This invention relates to a remote monitoring and controlling system wherein phenomena which have occurred in a plurality of slave stations have their time relations sequenced as a whole and are then processed and recorded.
In recent years, advances in the administration of an electric power system have made it necessary to exactly record and analyze the operating times and operating sequence of protective devices and various equipment in electric power stations over a large area.
In this regard, a conventional remote monitoring and controlling system includes a primary clock in a master station and secondary clocks in respective slave stations synchronization between the clocks is cyclically carried out to hold the time differences of all the clocks within a prescribed value. Signals representative of operations of certain protective devices and equipment are transmitted from the slave stations to the master station with the time data of the respective secondary clocks included. In the master station, the signals are sequenced on the basis of the included time data and then displayed and recorded.
In this case, the primary clock and the secondary clocks need to be synchronized. Since, however, the number of transmission channels between the master station and the slave stations is limited and delays are involved in the signal transmission, a special contrivance is necessary for the synchronization.
FIG. 3 is a block diagram which shows a prior-art remote monitoring and controlling system disclosed in Japanese Patent Application Laid-open No. 69496/1982. Referring to the figure, the system consists of a master station 0 and slave stations 1, 2, . . . . The master station 0 includes a display panel 3 which displays the status of each of the slave stations 1, 2, . . . , a console 4 which issues control commands to the respective slave stations 1, 2, . . . , a typewriter 5 which records operations and data, a processing circuit 6, an output circuit 7 which connects the display panel 3 to the processing circuit 6 through a bus, an input circuit 8 which connects the console 4 to the processing circuit 6 through the bus, a typewriter control circuit 9 which connects the typewriter 5 to the processing circuit 6 through the bus, a primary clock 10 which is connected to the processing circuit 6 through the bus, a code sending/receiving circuit 11 which is connected to the processing circuit 6 through the bus, and a modem 12 which is connected to the code sending/receiving circuit 11.
The slave station 1 includes a processing circuit 13a, a code sending/receiving circuit 14a which is connected to the processing circuit 13a through a bus, a modem 15a which is connected to the code sending/receiving circuit 14a, an input circuit 16a which is connected to the processing circuit 13a through the bus, an output circuit 17a which is connected to the processing circuit 13a through the bus, and a secondary clock 18a which is connected to the processing circuit 13a through the bus. The slave station 2 is identical in arrangement to the slave station 1, and the corresponding constituents of the former are respectively assigned the same symbols as the latter except for suffixes b replacing a.
Further, a signal transmission line 19a connects the modem 12 of the master station 0 and the modem 15a of the slave station 1. A signal transmission line 19b connects the modem 15a of the slave station 1 and the modem 15b of the slave station 2.
Next, the operation will be described. As illustrated in a timing chart of FIG. 4, the master station 0 sends a data request code "DRQ.sub.2 " to the slave station 2 by way of example. This data request code "DRQ.sub.2 " reaches the slave station 1 after a transmission delay time .tau..sub.1, and reaches the slave station 2 after a transmission delay time .tau..sub.2. When the processing circuit 13b of the slave station 2 identifies that the data request code "DRQ.sub.2 " that has arrived is directed to its own station, it returns the data "DOT.sub.2 " reporting its latest status. The processing circuit 6 of the master station 0 stores the data, and displays it on the display panel 3 through the output circuit 7. By repeating such operations, the master station 0 monitors and controls the slave stations 1, 2, . . . .
When the time for time synchronization approaches (for example, 0 a.m. to which the time is set once a day), the processing circuit 6 of the master station 0 sends the respective slave stations 1, 2, . . . a time setting code "TSE" into which the time t.sub.0 to be set (for example, 0 hour 0 minute 0 second 0 millisecond a.m.) has been encoded. When the respective slave stations 1, 2, . . . receive the code, the processing circuits 13a, 13b, . . . temporarily store the aforementioned time t.sub.0 to be set and wait for the subsequent arrival of a setting command signal "SET" . The processing circuit 6 of the master station 0 monitors the output of the primary clock 10 and transmits the setting command signal "SET" when the time t.sub.0 to be set has been reached. The setting command signal "SET" arrives at the slave stations after the transmission lags, namely, at the slave station 1 at a time t.sub.0 +.tau..sub.1 and at the slave station 2 at a time t.sub.0 +.tau..sub.2. Upon the arrival of the setting command signal "SET," the processing circuits 13a, 13b, . . . of the respective slave stations 1, 2, . . . add the transmission delay times .tau..sub.1, .tau..sub.2, . . . previously stored therein on the basis of measurements, to the value t.sub.0 of the time setting code "TSE," and they set the corresponding secondary clocks 18a, 18b, . . . to the resulting times t.sub.0 +.tau..sub.1, t.sub.0 +.tau..sub.2, . . . . Thus, the times of the secondary clocks 18a, 18b, . . . of the respective slave stations 1, 2, . . . are conformed to the time of the primary clock 10 of the master station 0.
Since the prior-art remote monitoring and controlling system is constructed as described above, the respective secondary clocks 18a, 18b, . . . must be kept exactly synchronous with the primary clock 10 in order that phenomena that occur in the slave stations 1, 2, . . . may have their time relations sequenced before they are processed. The synchronizing operation requires complicated control processes in which the transmission lag times must be taken into consideration. The synchronizing operation must be performed through the signal transmission lines 19a, 19b, . . . which have limited transmission capacity.